I'm going to start off by saying I'm a mechanical engineer, so I might need things explained to me as they would be to a child.

I am looking to build two H bridge motor controllers to control both direction and speed for 2 motors with a stall current of 14.5 A, although I expect to be running them much more in the 2-5 A range. I know what an H bridge is and how to operate one, but I am trying to avoid spending a huge grip on something like one of those $50-60 Pololu Dual motor controllers or a $70 Sabertooth Dual motor driver. I also want to learn some new stuff about electrical engineering instead of just buying something.

I understand how an H bridge is supposed to work, especially in the context of a motor with regard to direction, braking, free running, but I have discovered through laughably unsuccessful testing that Darlington Power Transistors are probably the wrong way to go. I'm looking into MOSFETs, of which I know nothing except that they are easier to break (?), but they seem to be able to handle greater currents (despite BJT's being the so called "current driven transistors" and FETs being the "voltage driven transistors"). Anywho, I am looking to control my robot with an Arduino's Digital I/O and PWM pins. My concern is that while looking at datasheets for power MOSFETs on Jameco.com is that all of the threshold voltages show a range of 2-4V with 2.9 typical. Would I kill one of these things with a 5V (or 4.5-4.9V what ever the range is) Arduino signal? Is there a way to bring the Voltage down? I wouldn't really have a clue since the extent of my electrical circuit analysis pretty much stops at analyzing ideal OP Amp circuits and normal, useless, KCL KVL RCL circuit analysis. Does anyone know of any good (free) online guides or resources I could look into for designing moderately high current H bridges, or at least something on using MOSFETs are switches? Any help would be graciously appreciated.

Don't worry so much about the multi direction and speed control part, I'm just trying to figure out what signals to send to a MOSFET and how to condition them from an Arduino.

At 14-15amp range, you need fairly beefy mosfets and you will need to use a mosfet gate driver for those things at moderate switching frequencies (1k or up).

Why so high of frequencies? I noticed the Pololu and Sabertooth boards mentioned going up to ridiculously high PWM frequencies over 20kHz so you can avoid a high pitched whine, but is the Arduino standard ~500Hz PWM frequency insubstantial? Also I am looking at beefy MOSFETs, like this onehttps://www.jameco.com/Jameco/Products/ProdDS/210542.pdf. Looking at the maximum rating on the drain, 72A peak, 18A continuous I should be fine right?

I know how to Google lol, and I know what an H bridge is. My questions are more about using MOSFETs. Yes I Googled that and I can't find any good resources on circuit designing or component selection for MOSFETS either. That's why I'm asking here is someone can either explain this kind of stuff or knows of any good resources.

Honestly, people tend to think this stuff is trivial, and can be explained in a sentence or two, but with h-bridges in the 15A range, you have no idea what you're getting into.

You would do well to heed this advice. Designing an h-bridge that works properly and doesn't self-destruct after you get past about 5 amps or so of current-capability gets difficult quickly. If your goal is to learn how to design and build such an h-bridge, and money isn't an issue, then go for it; you will learn a lot. If, however, your goal is to implement an h-bridge for some other project, you will probably find it cheaper to just purchase such an h-bridge that uses a proven design. Otherwise, in testing/experimenting with building your h-bridge, you may find yourself blowing expensive MOSFETs left and right - ultimately spending more money than if you had bought a properly-sized h-bridge in the first place...

I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

A discrete h-bridge is fairly difficult to design, primarily to get it to switch fast and to allow for the appropriate dead time.

If you are starting now, I would recommend that you look into h-bridge drivers (high/low-side drivers). They come in many different shapes, depending on your needs.

Generally, you need to decide if you want to have braking / motor hold; if you want to be able to drive high/low sides separately; if you allow enable signaling or not; your desired voltage ratings and if you want under/over voltage protection, etc.

Many firms offer such chips and they are generally easy to use. IRF, Fairchild and On would be good starting points.